Underground piping systems for steam and hot water heating of buildings in an area such as a city, campus, or the like, from a central heating plant serving the entire area, were known in old Europe and have been used in the United States primarily in institutional settings since about 1900. More recently, since about 1947, central cooling stations too are known and used in American cities such as Chicago. Insulation has been used about the pipes in such systems, including simple enclosed air spaces in conduits and tunnels and also insulating materials such as mineral wool, fiberglass batting, and foams of polyurethane and the like. Over time, however, the insulation in many such systems has deteriorated because of moisture getting into the insulation from condensation or ground water (as from flooding) as well as from thermal movement of the pipes during heating-up and cooling-down cycles. Various inventions are directed to replacing that insulation in place, such as U.S. Pat. No. 5,732,742.
Air space or insulation is used about the central pipe and within the outer conduit in sections of pipe built for new and replacement installations of underground piping systems. For instance, among U.S. Pat. No. 4,240,850 shows assembly of a sleeve creating an air space fillable with foam insulation about an inner pipe, using circumferentially-extending, apertured spacers; U.S. Pat. No. 3,877,136 applies foam insulation to and then forms a spiral-walled tube about an inner pipe; and U.S. Pat. No. 3,709,751 covers a foam layer about a pipe with a thin plastic sleeve. Installations with multiple pipes within larger outer conduits and in tunnels are known.
High-performance polyisocyanurate and similar foams for use in this application are known from U.S. Pat. No. 4,904,703 and the '742 patent, above.